Cross slope control for large mobile machines such as canal trimmers and liners and road pavers, although it is not absolutely necessary, offers solutions to age old construction problems if the cross slope control can be done accurately and effectively. To understand the problems that these cross slope controls solve, some attention must be devoted to the construction and transport of these machines.
Typically, large mobile machines for either trimming or lining of canals, the paving of roads, mobile conveyor or support frames have a large supporting steel frame(s) or structures. The large supporting steel frame is supported by conveyance equipment such as crawler tracks or wheels. Suspended from the steel frame is either paving, fine grading, trimming, conveying or lifting equipment. For example, in the preferred embodiments illustrated herein, four crawler tracks are utilized. In all cases, the frame includes four jacking columns--one for varying the elevation of each corner of a machine.
The elevation of the crawler tracks with respect to the large steel supporting frame is individually variable with hydraulically powered jacking columns. Specifically, by individually adjusting the elevation of the large supporting steel frame with respect to each of the crawler tracks, the elevation of the underlying trimming, or pavement can be controlled. In the case of strictly torsion control, each end of the steel frame can be held at the same attitude (relative to each other) preventing damage to the frame from unwanted torsion. In the case of the four crawler track machine, one support point is vertically adjusted from each crawler track. In the case of the two track machine, one support point in front and one support point at the rear of each supporting side bolster are vertically adjusted independently in relationship to a single crawler track.
In all cases except for torsion control, the machine requires a specified path (line) of travel and a reference to grade. These references are normally provided by one or more guide wires; however, line reference is sometimes provided off the edge of a previously poured slab and grade is sometimes referenced off the surface of an accurately placed, previously poured slab or trimmed sub-grade. These guide wires are accurately surveyed into place along the specified path of travel at an elevation that can be used as a reference to grade.
In many applications, guide wires are placed on both sides of the machine. The placement and maintenance of the guide wires can be expensive and, in some cases where space is limited, can cause an obstruction or interference. For example, placing guide wires on both sides of a road is roughly twice as expensive as placing such wires on one side of a road. Further, wires on both sides of a road can interfere with the required paving; trucks transporting concrete to the paving site can be severely restricted in entrance to and exit from a paving site bounded on both sides with guide wires, which causes delay. The wires on both sides can also severely limit the delivery of material to or removal of material from the machine. Also in some cases, there is simply not enough room on one side of the machine to place a guide wire and also have the necessary room for the machine crawler tracks to pass in the path provided.
Recognizing this, the prior art has developed systems for using one wire on one side of the road or pavement and utilizing cross-slope controls. The use of such cross-slope controls can best be understood with respect to a section of roadway having super elevation or banking on a curved section of the road.
It is well known that if a road has a slope extending angularly upward toward the outside of a curve, the slope of the pavement counteracts the centrifugal acting on a vehicle traveling over the curve in the road. The amount of the slope utilized is a function of the radius of curvature of a curve and the designed speed of the road. This slope is commonly referred to as "super elevation."
Assuming that only one guide wire is utilized, the large mobile paving or grading machine must reference its alignment (line) from that one wire and reference any cross-slope from the same single wire. In the prior art, these cross slope controls have included so-called "torsion bar controls" and "multiple cross slope controls."
In what follows, we present a detailed analysis of the weaknesses of the prior art. We are unaware of these weaknesses being cogently set forth and discussed. Accordingly, and in so far as recognition of the problems to be solved constitutes invention, we claim invention in the recognition of these problems as well as their solution:
Torsion bar controls utilize only one of the two transverse beams for the required cross slope control. This transverse beam is provided with a slope sensor that detects the angle of the transverse beam with respect to gravity. By adjusting the elevation of the cross slope side of the machine relative to the reference side of the machine, the slope is changed on the transverse beam to match the desired cross slope. Such a cross slope sensor may be found in the SF-350 Two Track Slipform Paver manufactured by the CMI Corporation of Oklahoma City, Okla., USA.
In addition to the required sensing of the cross slope, it is also required that the attitude or pitch on the reference side of the machine be relayed to the cross slope side of the machine. This is accomplished by CMI's "torsion bar" control. Specifically, a torsion bar is fastened rigidly to the reference side of the machine by means of an actuating arm (lever). This torsion bar extends from the reference side of the machine to the cross slope side of the machine. This extension of the torsion bar occurs through supporting bearings to the cross slope side of the machine. At the cross slope end of the torsion bar, an actuating arm extends from the torsion bar and is connected with a threaded adjusting link to an elevation control sensor to control the elevation of the front jacking column of the cross slope side of the machine. Attitude or pitch changes in the reference side of the machine cause the torsion bar controlled lever arm to vary the attitude or pitch of the cross slope side of the machine. Any adjustment of the attitude differential between the reference side of the machine and the cross slope side of the machine must be accomplished by manually adjusting the threaded adjusting link.
This control produces less than completely satisfactory results. First, for the torsion bar to function with absolute accuracy, it must be assumed that the large supporting steel frame is essentially rigid. This assumption is incorrect. For example, modern paving machines weigh in the range of 100,000 pounds. Even under static conditions, the large steel supporting frame of beam type construction will deflect under load. Moreover, where each of the crawlers encounters changes in elevation, the large steel supporting frames bend and deflect. As the large steel supporting frames bend, the reference that the torsion bars require to maintain the cross slope side level becomes distorted. Thus, because deflection/bending increases as the frame span increases, one can reason that the wider the paving width, the more distorted or inaccurate the cross slope becomes. Variation from the desired level condition of the paving or fine grading results.
Secondly, the crawler tracks propelling such machines often come out of synchronization. For example, the reference side of the machine can be in advance of the cross slope side of the machine while the machine is walking ahead or paving. As a result, the large steel supporting frames often "parallelogram" or change their shape when viewed in plan. When this occurs, the torsion bar is subjected to distortion. Thus, both the large steel supporting frame from which reference must be taken and the torsion bar itself are subjected to distortion and resulting inaccuracy.
Thirdly, there is the distortion and resulting inaccuracies related to the construction of the torsion bar itself. Because of the variable widths that the large supporting steel structure of the machine must assume, the torsion bar must have splices or joints in it so it can be adjusted in length. If these joints are not tight or the torsion bar is not of sufficient section, backlash can occur. In other words, a torsional (angular) movement on the reference side of the machine does not accurately translate into the same angular movement on the cross slope side of the machine.
Finally, even though the primary purpose of the torsion bar system is to keep both sides of the machine parallel with each other, "exact parallelism" is not always desirable. For instance when a highway is approaching a banked curve, one side of the machine may remain at a constant elevation and attitude on the inside of the curve while the other side of the machine must travel on an inclined path while it approaches the high, outside of the banked curve. Since the pavement surface is actually "warped" through this transition from a straight-away to a banked curve, it follows that the paving machine must also be slightly warped (within the limits of its flexibility) to produce a smooth, uniform paved surface. With the torsion bar system it is not feasible to make required differential attitude adjustments to control the warp of the machine frame while operating the paver; thereby, the resulting smoothness quality of the paved surface is adversely affected.
The "multiple cross slope control" is an alternative scheme of cross slope control. In such a control system, the reference side of the machine is provided with two separate transverse beams extending across the machine to the cross slope side of the machine. Typically, one transverse beam is at the front of the machine and the remaining transverse beam is at the rear of the machine. Cross slope sensors for detecting the slope of each of the two transverse beams with respect to gravity are provided.
Operation is easy to understand. As the machine tracks the intended course or alignment of the paving or fine grading, the cross slope sensor on each transverse beam measures the cross slope of each transverse beam. An "on board" computer (microprocessor) then compares the preset slope to the measured cross slope positions and thereafter controls the elevation of the respective forward and rear portions of the cross slope side of the machine by means of changes in the crawler track elevation to bring the machine back to the desired cross slope. This system is fully described in Snow U.S. Pat. No. 3,637,026 issued Jan. 25, 1972.
This system has its own difficulties. First, modern fine graders and pavers can be configured in relatively short and/or narrow configurations. These "compact" configurations can subject the supporting frames to high stresses during changes in crawler track elevation. In short, where the front transverse beam requires an elevation substantially different from the rear transverse beam, twisting of the frame with distortion of the resulting reference beams results. And where the frame is short in the direction of travel or narrow across its width, this tendency is aggravated. Moreover, if only a single transverse beam is used, supported on both ends as described above, the multiple cross slope does not work.
Secondly, such multiple cross slope control machines tend to relay changes in elevation in a loop around the machine. Change in elevation is typically sensed first at the leading portion of the machine at one crawler track. This change is relayed across the machine by detecting the slope of the transverse beam and varying the elevation of the front portion of the frame with respect to the crawler tracks. Unfortunately, the large steel supporting frame is of sufficient torsional rigidity to impart some of this correction through the frame to the rear transverse beam and rear cross slope sensor. Thereafter, and depending upon the elevation adjustment of the front portion of the machine, change is detected at the rear crawler track. This change--induced by adjustment to the forward portion of the machine--can be opposite to the correction of the forward portion of the frame. Thus a cycle of adjustment occurs with elevation changes in effect being relayed around the large steel supporting frame of the machine. This cycle of adjustment leads to further inaccuracy of the placed concrete or trimmed grade. Thirdly, large mobile machines including, but not limited to, those for the paving of roads, fine grading, and bulk material handling have a large supporting steel frame or structure. The large supporting steel frame is supported by conveyance equipment such as crawler tracks or wheels. During transport, accurate control of elevation is critically important. By way of example, it is known that these machines with rigid frames can encounter steep changes in elevation due to uneven ground. Where elevation is not precisely controlled, frame bending occurs often accompanied by splitting of welds and bending or buckling of frame members.
As a final note to the background of this invention, the reader should understand that the accurate control of ultimately placed paving material--either in a canal or on a roadway--is of vital importance to the contractor involved. Generally the smoother the surface that is trimmed or placed, the lower the concrete or placed material losses. Zero material losses means that the actually placed section is equal to the theoretical section. Moreover, for obvious reasons, smooth road surfaces are desirable to the motoring public. From an engineering standpoint, smoother roads last longer. For example, in the case of concrete or asphalt roadways, the finished product is measured by an instrument called a "profilograph." The profilograph measures the smoothness of the road surface. Dependent upon the measurements of these devices, incentive bonuses are either paid or not paid to the contractor for concrete placement. These so-called bonuses can be very large; thus, smoothness is critical to the profitability of the contract. Thus, to be able to realize the benefits of a cross slope control system without sacrificing the trimmed or paved surface smoothness quality is economically advantageous.